1. Field of the Invention
The present invention relates to an optical pickup device for optically reproducing information recorded on an optical storage medium such as CD (Compact Disc), DVD (Digital Versatile Disc) or LD (Laser Disc), and for optically recording information on the optical storage medium.
2. Description of the Related Art
An optical pickup device reproduces information recorded on an optical recording medium by irradiating the optical recording medium with laser light, and detecting reflected light (return light) from the optical recording medium. The laser light is focused on an information recording layer of the optical recording medium by an objective lens. It is necessary to prevent the position of a focused spot of the laser light from deviating from the focal position due to fluctuations such as an oscillating surface of the optical recording medium. For this purpose, the optical pickup device is equipped with a focus servo mechanism for driving the objective lens in real time to compensate for fluctuations of the optical recording medium.
The focus servo mechanism conducts a feedback control (i.e., focus servo control) which involves detecting return light from an optical recording medium to generate a focus error signal using the detected signal, and applying the focus error signal to an actuator for driving an objective lens. Due to differences in surface reflectivity and the like depending on the type of optical recording media, an initial adjustment must be done prior to the execution of the focus servo for adjusting the gain for the focus error signal, and the like in accordance with a particular optical recording medium.
Patent document 1 (Laid-open Japanese Patent Application No. 10-31828) discloses an exemplary initial adjusting method for a focus servo. This initial adjusting method applies a sawtooth voltage to a focus coil of an actuator a plurality of times to reciprocally move an objective lens. The reciprocal movements result in a focus error signal which has a sigmoidal curve or an inverted sigmoidal curve (hereinafter referred to as the “S-curve”) when the objective lens passes a focal position. The amplitude of the S-curve of the focus error signal is measured to determine a proper gain for a variable gain amplifier in accordance with the result of the measurement. FIG. 1 schematically shows the waveform of an FD signal representing a sawtooth voltage, and the waveform of a focus error signal (hereinafter referred to as the “FE signal”) corresponding thereto. During a period in which the FD signal increases in level, the objective lens is moved in a direction closer to the optical disc, whereas during a period in which the FD signal decreases in level, the objective lens is moved in a direction away from the optical disc. As shown in FIG. 1, the S-curve of the focus error signal appears in a very narrow range near a focal point.
However, the initial adjusting method described in the aforementioned patent document 1 is likely to cause the objective lens to collide with the optical disc. Generally, the sawtooth voltage is set at a frequency in a range significantly lower than a servo bandwidth due to the characteristics of the actuator. Also, the actuator has a very high drive sensitivity corresponding to the frequency of the sawtooth voltage. Therefore, if the reciprocal motion of the objective lens largely fluctuates in amplitude due to variations in the drive. sensitivity and drive gain of the actuator, the objective lens can collide with the optical disc.
Also, in recent years, there is an ongoing tendency to shorten the wavelength of laser light and increase NA (numerical aperture) of the objective lens for reducing the spot diameter of focused laser light to achieve a higher recording density. Since this tendency is accompanied with a shorter working distance between the objective lens and optical recording medium, the objective lens is more likely to collide with the optical disc, resulting in requirements for highly accurate initial adjustments.